Strawberry Sparkler

I guess I should have tasted this before I served it because it was so bitter. My frozen strawberries must not have been as sweet as everyone else's. I imagine adding sugar would have helped but I just assumed it would be good and served it per the instructions. I make champagne punches all the time and have never had a bust until this one. My guests politely drank half of their glasses and left them on the tables. I won't bother to try this one again!